add docs for different git providers

[krancour]

Different git providers have different rules around the username + password/token used for repository access.

For example, some require you to use the valid username associated with the token and for others the username should be "git" or can be omitted entirely.

As support is added for each git provider, helpful provider-specific details should be added to the docs as well.

See:

• add support for opening PRs on bitbucket #43
• add support for opening prs on Azure Devops #44
• add support for opening PRs on GitLab #45
• add support for opening prs on GitHub Enterprise #46

[senare]

Would be nice with instructions for generic GIT i.e. HTTP/SSH style repo

Also self hosted alternatives such as

• Gitea or GOGS
• soft-serve

[krancour]

@senare these instructions should mostly work for any HTTP/S repo on any git hosting platform. (SSH is a different story atm -- it's only half-implemented.)

https://kargo-render.akuity.io/how-to-guides/docker-image

It's really just entrypoints and pull request functionality that needs to be implemented differently from one git hosting platform to the next.